Add "pithos" as a player compatible with the sound applet.

I added "pithos" to "let compatible_players = [" in the sound applet's "applet.js" out of curiosity, and (to my surprise) the album art, skip forward button, and play/pause button all worked great with no other changes.

For the unfamiliar, pithos is a GTK+ client for Pandora Internet Radio available in the default repos.

I have _never_ made a pull request before, so I apologize in advance if I'm not following correct procedure.  Thanks! :)

![Pithos in Sound Applet Screenshot - Cinnamon 1 6 7 nadia](https://f.cloud.github.com/assets/1298191/14336/7eb5dfe8-4638-11e2-84ab-866dcd217a73.png)

Edit: I forgot to mention that the "stop" and "skip back" buttons are shown but have no function with pithos if somebody with more familiarity would like to correct that.